1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance tomography apparatus of the type having a first magnet system which produces a magnetic field having a first field strength for pre-polarization of the nuclear spins in an examination subject, and a second magnet system which produces a magnetic field having a second field strength that is significantly lower compared to the first field strength, with a gradient coil system for generating magnetic field gradients and a radio-frequency means for the excitation of nuclear spins as well as for the read-out of the arising nuclear magnetic resonance signals being allocated to the second magnet system.
2. Description of the Prior Art
A nuclear magnetic resonance tomography apparatus of the above type is disclosed, for example, in U.S. Pat. No. 5,296,811, as well as in A. Macovski, S. Conolly, "Novel Approaches To Low-Cost MRI" MRM 30: 221-230 (1993).
In a conventional nuclear magnetic resonance tomography apparatus, the nuclear spins are aligned in the direction of the magnetic field by a magnetic field produced by a magnet that accepts the body of the examination subject. The nuclear spins are excited by radio-frequency pulses and the arising nuclear magnetic resonance signals are read out in the same magnet system. A tomogram of the examination subject is subsequently reconstructed from the signals acquired in this way.
The magnet that is employed must have a comparatively high field strength so that an adequate signal-to-noise ratio is achieved. The signal strength of the nuclear magnetic resonance signal increases with increasing field strength. Typically, field strengths between 0.2 T and 4 T are employed.
Further, high demands are made with respect to the uniformity of the magnetic field, since spatial distortions and artifacts otherwise occur in the image that is acquired. In order to achieve the acquired homogeneity, magnets having a coil arrangement, which are typically implemented as superconductive magnets, must have a certain minimum structural length. If pole shoe magnets are used, a specific ratio of pole shoe diameter to pole shoe spacing cannot be downwardly transgressed for the same reason. Moreover, the required energy in the magnet increases with the spacing between the pole shoes.
Such magnets become extremely complicated due to the combination of the aforementioned conditions, namely, the combination of achieving high magnetic field strength with utmost uniformity. Typically, they represent by far the most expensive individual component of a nuclear magnetic resonance tomography system. Access to the patient during the examination is greatly limited in practice, further, because of the same demands. Pole shoe magnets, particularly according to the C-bend design, in fact allow significantly better access to the examined patient than do coil magnets. Due to the aforementioned conditions, however, the spacing between the pole shoes must be optimally small and the pole shoe surface must be made as large as possible, making interventional examinations more difficult.
One can manage with significantly simpler magnet arrangements when a pre-polarization of the examination subject is implemented, according to the aforementioned publications. In this case, the problem of the high field strength and the problem of high uniformity have been divided: a high field strength, namely, is only required for the pre-polarization but the demands made of the uniformity are relatively low since inhomogeneities are not expressed in local distortions or artifacts, but at most in terms of (correctable) image shadowings. The demands made of the uniformity are in fact high for the second magnet system in which the actual image acquisition ensues; however, one can manage with relatively low magnet field strengths since the signal-to-noise ratio is defined by the pre-polarization. Magnets that must satisfy only one of the demands with respect to uniformity or high field strength, however, can be built significantly more inexpensively. The aforementioned problems of accessibility can also be largely avoided.
Nuclear magnetic resonance tomography systems with pre-polarization, however, have not prevailed in practice, particularly since the image quality of conventional systems cannot be entirely achieved.
The reference, N. Albert et al., "Magnetic Resonance Imaging Using Hyperpolarized Xe", "American Journal Of Electro Medicine", December 1994, pgs. 72-80, discloses a method for MR imaging with hyperpolarized inert gases. The polarization does not occur on the basis of a strong magnetic field, but by laser-induced optical pumping. The hyperpolarized inert gas, for example .sup.129 Xe or .sup.3 He, is then administered to a patient by inhalation. The imaging occurs in a conventional nuclear magnetic resonance tomography apparatus. The high magnetic field of a classic nuclear magnetic resonance tomography apparatus thus not required at all, and is more likely to be disruptive due to its limited patient accessibility.